Add a contributing.md file

Author: marijnh

CONTRIBUTING.md

@@ -0,0 +1,96 @@
+# How to contribute
+
+- [Getting help](#getting-help)
+- [Submitting bug reports](#submitting-bug-reports)
+- [Contributing code](#contributing-code)
+
+## Getting help
+
+Community discussion, questions, and informal bug reporting is done on
+the [discuss.CodeMirror forum](http://discuss.codemirror.net).
+
+## Submitting bug reports
+
+Report bugs on the
+[GitHub issue tracker](http://github.com/codemirror/dev/issues).
+Before reporting a bug, please read these pointers.
+
+- The issue tracker is for *bugs*, not requests for help. Questions
+  should be asked on the [forum](http://discuss.codemirror.net).
+
+- Include information about the version of the code that exhibits the
+  problem. For browser-related issues, include the browser, browser
+  version, and operating system on which the problem occurred.
+
+- Mention very precisely what went wrong. "X is broken" is not a good
+  bug report. What did you expect to happen? What happened instead?
+  Describe the exact steps a maintainer has to take to make the
+  problem occur. A screencast can be useful, but is no substitute for
+  a textual description.
+
+- A great way to make it easy to reproduce your problem, if it can not
+  be trivially reproduced on the website demos, is to submit a script
+  that triggers the issue. The easiest way do do that is our
+  [sandbox](https://codemirror.net/try/).
+
+## Contributing code
+
+Code written by "AI" language models (either partially or fully) is
+**not welcome**. Both because you cannot guarantee it's not parroting
+copyrighted content, and because it tends to be of low quality and a
+waste of time to review.
+
+- Make sure you have a [GitHub Account](https://github.com/signup/free)
+
+- Fork the relevant repository
+  ([how to fork a repo](https://help.github.com/articles/fork-a-repo))
+
+- Create a local checkout of the code. You can use the
+  [dev repository](https://github.com/codemirror/dev) to
+  easily check out all core modules.
+
+- Make your changes, and commit them
+
+- Follow the code style of the rest of the project (see below).
+
+- If your changes are easy to test or likely to regress, add tests in
+  the relevant `test/` directory. Either put them in an existing
+  `test-*.js` file, if they fit there, or add a new file.
+
+- Make sure all tests pass. Run `npm run test` to verify tests pass.
+
+- Submit a pull request ([how to create a pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork)).
+  Don't put more than one feature/fix in a single pull request.
+
+By contributing code to CodeMirror you
+
+ - Agree to license the contributed code under the project's [MIT
+   license](https://github.com/CodeMirror/dev/blob/main/LICENSE).
+
+ - Confirm that you have the right to contribute and license the code
+   in question. (Either you hold all rights on the code, or the rights
+   holder has explicitly granted the right to use it like this,
+   through a compatible open source license or through a direct
+   agreement with you.)
+
+### Coding standards
+
+- TypeScript, targeting an ES2018 runtime (i.e. don't use library
+  elements added after ES2018).
+
+- 2 spaces per indentation level, no tabs.
+
+- No semicolons except when necessary.
+
+- Follow the surrounding code when it comes to spacing, brace
+  placement, etc.
+
+- Brace-less single-statement bodies are encouraged (whenever they
+  don't impact readability).
+
+- [getdocs](https://github.com/marijnh/getdocs-ts)-style doc comments
+  above items that are part of the public API.
+
+- CodeMirror does *not* follow JSHint or JSLint prescribed style.
+  Patches that try to 'fix' code to pass one of these linters will not
+  be accepted.